Air-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene

Abstract

Phytoplankton productivity and community structure in the East China Sea (ECS) play an important role in marine ecology and carbon cycle, but both have been changing rapidly in response to recent oceanic and atmospheric circulation changes. However, the lack of long-term records of phytoplankton productivity and community structure variability in the region hinders our understanding of natural forcing mechanisms. Here, we use the phytoplankton biomarker (brassicasterol, dinosterol and alkenones) contents as well as the ratios between these biomarkers in three sediment cores from the ECS shelf to reconstruct the spatiotemporal variations of productivity and community of diatoms, dinoflagellates and coccolithophores during the Holocene, respectively. During 9-7 ka, the ECS shelf was characterized by low phytoplankton productivity with low coccolithophore contribution, caused by the oligotrophic condition mainly owing to the restricted Kuroshio Current (KC) intrusion under low sea-level conditions, thus the lack of nutrient input. Phytoplankton productivity generally increased during 7-4.6 ka, in response to the initial intrusion of the Yellow Sea Warm Current (YSWC, a branch of the KC), bringing nutrient from the subsurface KC to the upper layer of the ECS for phytoplankton growth. Phytoplankton productivity continuously increased during 4.6-1 ka, due to an enhanced circulation system (YSWC and Yellow Sea Coastal Current (YSCC)) driven by strong East Asia Winter Monsoon (EAWM). Significantly, high alkenone contents and coccolithophore contribution in the eastern core F11A was associated with its location closer to the warm and saline YSWC, which was suitable for coccolithophore growth. Beyond diagenetic processes which could partly account for higher biomarker contents near core tops, elevated phytoplankton productivity during the last 1 ka might be induced by more nutrient supply from the intensified circulation system driven by enhanced KC and anthropogenic activities. The latter also resulted in high dinoflagellate proportions in all three cores. These temporal and spatial changes of phytoplankton productivity and community structure in the ECS during the Holocene corresponded to different mechanisms by the air-sea interaction, providing insights into distinguishing natural forcing and anthropogenic influences on marine ecology.

New Insights Into Kuroshio Current Evolution Since the Last Deglaciation Based on Paired Organic Paleothermometers From the Middle Okinawa Trough

“…For the analysis of unsaturated alkenones and glycerol dialkyl glycerol tetraethers (GDGTs), samples were taken at 2‐cm intervals giving a total of 183 samples. Sample processing and instrumental analyses of alkenones and GDGTs were performed at the Ocean University of China following Z. Wang et al (2019) and Z. Yuan et al (2018). Briefly, approximately 3 g of freeze‐dried sample was ultrasonically extracted four times using 10 ml of dichloromethane/methanol (DCM:MeOH; 3:1, v /v) after adding internal standards (n‐C 24 D 50 , C 19 n‐alkanol, and C 46 GDGT).…”

“…Sample processing and instrumental analyses of alkenones and GDGTs were performed at the Ocean University of China following Z. Wang et al (2019) and Z. Yuan et al (2018).…”

Biomarker evidence of the water mass structure and primary productivity changes in the Chukchi Sea over the past 70 years

“…Abundant marine ecosystem-derived compounds were also detected in Core R07, including brassicasterol (24-methylcholesta-5,22E-dien-3b-ol), dinosterol (4a,23,24-trimethyl-5a-cholest-22-en-3b-ol), and C 30 diols, which are thought to be biomarkers for diatoms, dinoflagellates, and eustigmatophytes, respectively, and their concentrations can reflect the corresponding phytoplankton productivity (Schubert et al, 1998;He et al, 2013). Although brassicasterol can be produced by algae other than diatoms, such as dinoflagellates and haptophytes (Volkman, 1986;Ding et al, 2019), in an ecosystem dominated by diatoms, this compound is mainly produced by diatoms and could be used for reconstructing the diatom productivity (Wu et al, 2016;Wang et al, 2019). In the Chukchi Sea shelf, the phytoplankton community was dominated by diatoms, which account for at least 60% of the total population (Sergeeva et al, 2010;Lewis et al, 2019).…”

Responses of Marine Diatom-Dinoflagellate Competition to Multiple Environmental Drivers: Abundance, Elemental, and Biochemical Aspects

“…Equally important is biochemical composition in phytoplankton. Certain phytoplankton-produced biomolecules (biomarkers), such as fatty acids (FAs) and sterols have been applied as trophic markers for tracing food web structures ( Dalsgaard et al, 2003 ; Martin-Creuzburg and von Elert, 2009 ; Ruess and Müller-Navarra, 2019 ; Kohlbach et al, 2021 ) and as proxies for reconstructing phytoplankton community structure in the past and present-day ocean ( Schubert et al, 1998 ; Zimmerman and Canuel, 2002 ; Wu et al, 2016 ; Wang et al, 2019 ). Ocean-related global change has significant effects on the chemical composition of phytoplankton ( Guschina and Harwood, 2009 ; Hixson and Arts, 2016 ; Ding et al, 2019 ; Bi et al, 2020 ), e.g., showing up to a 83% increase in stoichiometric P:C in response to warming and a 76% increase in carbon-normalized contents of polyunsaturated fatty acids (PUFAs) in response to nutrient deficiency ( Bi et al, 2017 ).…”